Many vehicles which require four-wheel traction have been provided with an auxiliary hydraulic front wheel drive which works in conjunction with a conventional mechanical transmission providing the primary drive for the vehicle. Systems of this type have been particularly useful for earth moving equipment, tractors, self propelled farm machinery and other self propelled vehicles which normally include a hydraulic system for powering various vehicle accessories. U.S. Pat. Nos. 3,339,660 and 3,354,978 to Tadeusz Budzich disclose front axle fluid power drive systems employing fluid motors which are supplied by a variable displacement pump. These systems are controlled by an automatic pressure responsive control which varies the pump displacement to maintain a constant system pressure and a constant torque at the fluid motors.
Initially, most auxiliary hydraulic drive systems utilized a separate fluid pump and were totally independent of the hydraulic system employed to power vehicle accessories such as steering, brakes, earth moving blades and the like. However, auxiliary drive systems have been developed which use the pump source for the vehicle accessories rather than a separate and independent pump. Such a system is illustrated by U.S. Pat. No. 3,984,978 to Loren L. Alderson which discloses the use of a pair of variable displacement axial piston motors, each directly connected to the front wheel of a vehicle. The motors are pressure compensated at a level slightly below the pressure compensating level of the primary system pump for the accessories system, and a flow compensating control is provided to cause the motor swash plate to destroke and maintain a constant flow rate if the flow level through the motors exceeds a predetermined rate.
Often it is desirable to selectively connect the motors of the auxiliary front wheel drive system to the driven wheels by means of hydraulically activated clutches. In such cases, pressurized fluid supplied to each motor actuates a clutch associated with that motor so that the motor is drivingly engaged with the wheel. A system of this type is disclosed in U.S. Pat. No. 4,069,886 to John J. Campbell et al.
In the auxiliary hydraulic front wheel drive systems of the prior art, pilot operated spool valves are normally used in the pressure and flow control system for the front wheel variable displacement drive motors. These valves are continuously subjected to fluid pressure, even when the associated variable displacement motor is inoperative. With the motor inoperative, the fluid pressure applied to the stems of the spool valves in the control system is likely to cause foreign matter and slit to build up in the spool valves resulting in subsequent valve sticking.